FIG. 1 shows diagrammatically, in a schematic, the global architecture of an SBAS system according to the prior art. Such a system is adapted for producing augmentation data on the basis of measurements carried out on the navigation signals 101 emitted by a plurality of radio navigation satellites NAV. The measurements and data originating from the radio navigation satellites NAV are collected by a plurality of observation stations RIMS and then transmitted 102, at a given tempo, to one or more monitoring and processing centres CPF. These carry out, on the basis of the measurements received, an estimation of the differential corrections to be applied to the radio navigation signal and then elaborate augmentation messages, which include these corrections and are thereafter transmitted 103 to a terrestrial navigation station NLES. The station NLES receives the augmentation messages and transmits them 104 to an augmentation satellite SAT for subsequent broadcasting 105 to the users U and to the observation stations RIMS. The users U may be receivers situated on the ground or aboard a carrier, for example an aircraft. The augmentation messages are, for this purpose, integrated, in a manner similar to the navigation messages, into a navigation signal compatible with a satellite based radio-navigation system or GNSS (Global Navigation Satellite System). The augmentation satellite SAT may be a geostationary satellite or a satellite with high orbit HEO or else a satellite with inclined geostationary orbit of IGSO type (Inclined Geosynchronous Satellite Orbit). This is for example a satellite of the EGNOS (European Geostationary Navigation Overlay Service) European system. The station NLES performs integrity monitoring of the messages received, transmitted by the monitoring and processing centres CPF, with the messages broadcast by the augmentation satellite SAT on the downlink.
In a satellite based augmentation system, the observation stations RIMS, tasked with carrying out measurements on the radio-navigation signals received, are usually of limited number. For example, the EGNOS European SBAS system comprises only thirty-seven observation stations in its currently operational version and two other stations are presently undergoing deployment. Likewise, the WAAS American SBAS system comprises thirty-eight observation stations. A limited number of observation stations implies a limited availability in respect of the service associated with the SBAS system, in particular in the geographical zones where the service must be ensured but which comprise only few stations. Furthermore, the number of observation stations directly impacts the performance of the system in terms of precision of the modellings carried out on the basis of the measurements collected. In particular the modellings of the orbits of the satellites as well as the ionosphere that are provided by an SBAS system will be all the more precise the higher the number of stations.
To increase the number of available observation stations, the idea underlying the invention consists in using, in place of or as a supplement to the observation stations RIMS dedicated to the SBAS system, external stations whose prime function is not the carrying out of measurements destined for a processing centre CPF of an SBAS system. In particular, such external stations may be provided by bodies that manage collaborative networks of radio-navigation stations. For example, the bodies IGS (International GNSS Service), EUREF (European Reference) IGN (French national geographical Institute) respectively offer the collaborative networks IGS global network, EPN (EUREF Permanent Network) and RGP (Permanent Geodesic Network).
Such collaborative networks comprise a large number of GNSS stations, for example the RGP network comprises 300 stations, however the measurements provided by these stations are not guaranteed. If a station exhibits a problem in its operation and transmits an aberrant measurement, this may disturb the global operation of the system.
To be compatible with the integrity requirements demanded by an SBAS system, the data provided by the stations external to the said system, acting instead of the observation stations RIMS, must be monitored in order to comply with the specified integrity level.
The known solutions making it possible to perform integrity monitoring of the measurements received by the processing centre CPF are of two types.
A first solution consists in using an item of information about the quality of the radio-navigation signal received by the observation stations RIMS so as to filter the aberrant measurements. Such an item of information regarding quality is calculated by the internal observation stations RIMS of a standard SBAS system and transmitted to the processing centre CPF. However, stations external to the SBAS system do not integrate such a calculation and without an item of information regarding quality, the filtering method implemented by the processing centre CPF is not possible.
A second solution consists in using a monitoring function in parallel with the function tasked with detecting integrity defects as is conventionally done for example in a monitoring and processing centre of the EGNOS European augmentation system. Such a solution allows the use in confidence of an SBAS navigation message but results in a reduction in availability since it involves filtering all the measurements arising from one and the same satellite or originating from a given zone of the ionosphere.